FIG. 1 (prior art) is a simplified cross-sectional diagram of one type of so-called white LED assembly 1. Assembly 1 includes a lateral blue LED device 2. The active layer 3 of the blue LED device 2 emits light in all directions, and the light bounces randomly within the LED device. A substantial amount (about 50%) of light travels downward. If the light 4 traveling downwards is not reflected back upward so that it can then escape from the top surface of LED device, but rather if the light traveling downwards is absorbed by the die-attach adhesive or by the aluminum core PCB, then the light generation efficiency of the overall white LED assembly will suffer.
The structure of the lateral LED device entails a sapphire substrate 5 that is substantially transparent to the blue light. Accordingly, a reflector structure 6 is disposed on the backside (i.e., bottom side in the diagram) of the transparent substrate 5 to reflect light that was traveling in a downward direction. Reflector structure 6 reflects the light that travels downwards, passes this light back up and through the transparent substrate and through the epitaxial layers of the LED device. The reflected light then escapes the LED device and reaches phosphor 7 embedded in encapsulant, such as silicone. The phosphor absorbs some of the blue light and fluoresces, thereby re-emitting light of longer wavelengths including green, yellow and red light. The overall spectrum of light emitted from the overall LED assembly 1 is therefore said to be white light. This white light is the useful light produced by the assembly.
The reflector structure 6 can be a single layer of a highly reflective metal such as, for example, silver. Unfortunately, silver has attendant contamination and electromigration issues. For this and other reasons, LED devices such as the LED device 2 of FIG. 1 may have reflector structures involving a total internal reflection (TIR) layer 8, a Distributed Bragg Reflector (DBR) structure 9, and an underlying layer 10 of reflective metal. The combination of these layers is superior in terms of reflectivity to a single mirror layer of a highly reflective metal.
According to Snell's law, all of the light traveling from a material having a higher index of refraction toward a material having a lower index of refraction at an angle greater than the critical angle will be reflected back into the higher-index-of-refraction material without experiencing any energy loss. This mechanism is known as total internal reflection (TIR). The TIR layer 8 is fashioned to reflect blue light that is passing toward the reflector at angles greater than the critical angle. The lower two portions 9 and 10 of the reflector structure (the DBR and the reflective metal layer) are provided to reflect any remaining light that passes through the TIR layer.
In its simplest form, a DBR is a quarter wave stack of dielectric materials. The quarter wave stack consists of a stack of layers, where the material from which the layers are made alternates from layer to layer down the stack. The materials are selected such that the alternating layers have a high index of refraction, and then a low index of refraction, and then a high index of refraction, and so forth down the stack. For a given wavelength of light entering the stack from the top, the upper layer is made to have a thickness of one quarter of the wavelength, where this wavelength is the wavelength of the light when the light is passing through the layer. The wavelength λ, frequency f, and velocity v of light is given by the equation λ=v/f. When light leaves one medium and enters another medium, the speed and wavelength of the light may change but the frequency does not change. The material from which the upper layer is made therefore determines the speed of light v in the medium. The material therefore also influences the wavelength λ of the light in the upper layer.
Each material has an index of refraction η. The index of refraction η is the ratio of the speed of light in a vacuum to the speed of light in the medium. The wavelength of light in a medium is given by the equation λ=λo/η, where λo is the wavelength in a vacuum. Light traveling through air is traveling at close to the speed of light in a vacuum, so the wavelength of light in air is close to wavelength of the light in a vacuum. The design wavelength λo for the DBR is usually longer than the LED emitting wavelength when the reflectivity of the DBR for the light with incident angles between zero degrees and the critical angle is considered. For example, the optimal DBR design wavelength for a 450 nm LED is around 510 nm. The relationship QWOT=λo/4η is used to determine the quarter wavelength in the medium of a layer, where η is the refractive index of the material from which the layer is made. In this way, the refractive indices of the materials of the various layers of the stack are used to determine how thick each layer of the stack should be so that it is one quarter wavelength in thickness.
Light passes into the stack and through the upper layer, and then some of the light reflects off the interface between the upper layer and the next layer down in the stack. Part of the light proceeds down into the next layer of the stack to the next interface. If the interface is one from a low-index medium to a high-index medium, then any light reflected from the interface will have a phase shift of 180 degrees. If, on the other hand, the interface is one from a high-index medium to a low-index medium, then any reflected light will have no phase shift. Each interface causes a partial reflection of the light wave passing into the stack. The phase shifts, in combination with the thicknesses of the layers of the stack, are such that the portions of light reflecting off interfaces all return to the upper surface of the stack in phase with each other. The many reflections off the many interfaces all combine at the top of the stack with constructive interference. The result is that the Distributed Bragg Reflector has a high reflectivity within a finite spectral range known as the stop-band. Then lastly at the bottom of the reflector structure 6 is the layer 10 of reflective metal.
FIG. 2 (prior art) is a table that sets forth the thicknesses and materials of the various layers of the Distributed Bragg Reflector of the prior art LED device 2 of FIG. 1 based on a design wavelength of 510 nm. The π notation above the line between two rows indicates that the light reflected by the interface between the materials of the two rows is phase shifted by 180 degrees. The upper SiO2 layer has a thickness of 4101 angstroms and is the TIR layer 8. The DBR structure 9 includes three periods, where each period has a first layer of TiO2 that is 447 angstroms thick and a second layer of SiO2 that is 820 angstroms thick.
FIG. 3 is a diagram that shows the normal-incident reflectivity spectrum with the reflector design described in FIG. 2. The stop-band of the spectrum centers around 510 nm, and the short wavelength side of the stop-band is aligned to 450 nm. According to theoretical calculation, the reflectivity spectrum blue-shifts toward the short wavelength when the light incident angle increases from surface normal toward grazing angle to the reflector. The reflector was optimized to ensure high reflectivity for the light with wavelength of 450 nm over a broad range of incident angles. FIG. 4A is a diagram that charts the reflectivity of the reflector structure 6 versus the angle of incidence of light with a wavelength of 450 nm reaching a point 11 on the reflector. The light with incident angles between 0 and 58 degree are reflected by the DBR and the metal reflector, while the light with incident angle greater than 58 degree is reflected by the TIR layer. To evaluate the total reflectivity of the reflector with all incident angles, a normalized angular reflectance is defined. Referring to FIG. 4B, light is assumed to be transmitted toward point 11 on the reflector from all directions with a uniform angular distribution. The amount of light incident on the point that is reaching the point 11 with an incident angle θ is considered. Many different light rays may actually reach the point from this incident angle, where the light rays can be thought of as passing to the point in a cone shape. The upper lip of the cone 12 illustrated in FIG. 4B represents a circle of origination points for such rays for the incident angle θ. Accordingly, there is more light incident on point 11 for an incident angle of one degree than for an incident angle of zero degrees. This larger amount of light at larger angles is considered, and the corresponding total amount of reflected light is determined for angles zero (orthogonal) through 90 degrees (a grazing angle). The normalized angular reflectance is then calculated by integrating the angular reflectivity (FIG. 4A) with a sine dependence of incident angle and normalized to a perfect angular reflectivity spectrum. This analysis is performed for light of a given wavelength, for example 450 nm, to compare the performance of the reflector for blue light emitted by the LED in the white LED assembly FIG. 1. When analyzed this way, the prior art reflector structure of the LED device of FIG. 1 has a reflectivity of approximately 97 percent for incident blue light (having a wavelength of 450 nm). Accordingly, most all of the blue light 4 traveling downward is then reflected back up the reflector so that it can escape the LED device. The reflector structure involving DBR 9 is more effective than a simple mirror layer of a reflective metal such as silver.